$a = 0;
OUTER:
while( $a < 4 ){
   $b = 0;
   print "value of a: $a\n";
   INNER:
   while ( $b < 4){
      if( $a == 2){
          # terminate outer loop
          last INNER; 		#last后面没标签则跳出当前循环，如果有标签则跳出标签所在的循环 只能跳出到前面的label
      }
      $b = $b + 1;
      print "Value of b : $b\n";
   }
   print "\n";
   $a = $a + 1;
}